DataWeight and DataWeightProxy do behave different

[Peter]

Equality operator for DataWeight is implemented as

bool operator==(const DataWeight& lhs, const DataWeight& rhs)
{
	return lhs.data() == rhs.data();
}

while DataWeightProxy is implemented as

template<typename DataIterator, typename WeightIterator>
bool operator==(const DataWeightProxy<DataIterator, WeightIterator>& lhs,
		const DataWeightProxy<DataIterator, WeightIterator>& rhs)
{
	return lhs.data()==rhs.data() && lhs.weight()==rhs.weight();
}

I suggest DataWeightProxy should be reimplemented, but is it even needed since we allow implicit conversion DataWeightProxy => DataWeight?

[Peter]

(In [2122]) Remove implementation of operator==(DataWeightProxy?, DataWeightProxy?) and rather rely on implicit conversion to DataWeight? and its operator. This implies the behavior changes but that change is needed because the previous behavior was very unexpected.

closes #578

[Peter]

(In [2123]) refs #578. Mention change of DataWeightProxy? in NEWS